Many modern vehicles are equipped with advanced safety and driver-assist systems that require robust and precise object detection and tracking systems to control host vehicle maneuvers. These systems utilize periodic or continuous detection of objects and control algorithms to estimate various object parameters, such as the relative object range, velocity, direction of travel, and size. For example, radar devices detect and locate objects (i.e., targets), by transmitting electromagnetic signals that reflect off targets within a sensor's field-of-view. The reflected signal returns to the radar as echoes where they are processed to determine various information such as the round-trip travel time of the transmitted/received signals. However, when multiple targets are present, certain radar devices lack the angular and/or spatial resolution necessary to distinguish between multiple targets that are closely-located. In these cases, wherein multiple located targets are so closely located that they cannot be separated by range or angle, the targets may still be separated by a Doppler frequency if the Doppler resolution of the radar device is sufficiently high.
The Doppler effect manifests itself when there is a relative range rate, or radial velocity, between the host vehicle with the radar and the target. When the radar's transmit signal is reflected from such a target, the carrier frequency of the return signal will be shifted. Assuming a co-located transmitter and receiver, the resulting Doppler frequency shift is a function of the carrier wavelength and the relative radial velocity (range rate) between the radar and the target. When the target is moving away from the radar, the relative radial velocity, or range rate, is defined to be positive and results in a negative Doppler shift; when the target is moving towards the radar, the opposite occurs.
Advanced radar systems in use today may utilize a multiple-input multiple-output (MIMO) concept that employs multiple antennas at the transmitter to transmit independent waveforms and multiple antennas at the receiver to receive the radar echoes. In a “co-located” MIMO radar configuration, the antennas in both the transmitter and the receiver are spaced sufficiently close so that each antenna views the same aspect of an object such that a point target is assumed. In the MIMO receiver, a matched filter bank is used to extract the waveform components. When the signals are transmitted from different antennas, the echoes of each signal carry independent information about detected objects and the different propagation paths. Phase differences caused by different transmitting antennas along with phase differences caused by different receiving antennas mathematically form a virtual antenna array that provides for a larger virtual aperture using fewer antenna elements. Conceptually, the virtual array is created by interleaving between each of the transmitter Tx and receiver Rx antenna elements such that the elements in the virtual array represent Tx-Rx pairs for each of the transmitter Tx and receiver Rx antennas in the MIMO array. For co-located MIMO antennas, a transmit array having N transmitter antennas and a receive array having M receive antennas produces a virtual array having M×N virtual receiver elements. In other words, the waveforms are extracted by the matched filters at the receiver such that there are a total of M×N extracted signals in the virtual array.
In addition to generating and transmitting individual waveforms from each transmitter antenna, the transmitted signals may be encoded using various coding techniques. For example, each transmitter antenna may be configured to transmit a waveform with a different code. Thus, a transmit array will transmit N spatial codes that span the entire radar field-of-view, which generally ranges from 120° (+/−60° from boresight) to 180° (+/−90° boresight). The codes are transmitted over each of the N transmit antennas as a sequence of symbols. Because the received signal vector is a sum of the echo signals transmitted from all of the N transmit antennas, to achieve separation of the N transmission channels at the receiver, the number of symbols in a sequence (i.e., the number of transmitted codes) must be equal to the number of transmit antennas, N. Consequently, as the number of transmit antennas increases, so does the length of the codes and the repetition interval of each sequence. However, as the repetition interval increases, ambiguity arises with respect to velocity estimation (i.e., Doppler frequency).